Hi. I had asked this question on zope3 about a week ago or so an had no responses. I am hoping I can receive some general guidance on this issue. I am trying to determine the best structure for storing a large schema where some attributes are lists or dictionaries. There are about 70 attributes in the schema and I am trying to choose a structure that will not necessarily have to hold a bunch of empty space. I have a base schema of approximately 30 attributes and others that subclass from it with the largest being about 70.

I had originally thought of RDF at the onset and built a datastore with rdflib using a relational database. I chose this option because when ZODB gets larger it takes plenty of RAM. Problem here is the number of accesses to gather a complete object. It is pretty efficient from a storage perspective since if an object does not have particular attributes, you are not storing them and all items in the store are unique. I was not duplicating one piece of data. But say you wanted to present a page consisting of 20 items or do a search. Gathering this up takes much time when you are hitting a disk how many times to gather up just a single object so query times were unacceptable and loading rdf from outside sources also took a very long time. The data store grows into millions of records so you better have a pretty sweet rdb server with lots of RAM also.

I had dismissed a relational database on its own since the data does not lend to a row and I may want to add to the schema at some point in time which could mean some pretty ugly business this way. But then I saw the vertical example in the examples folder of SQLAlchemy that can do something to create dynamic fields as necessary to potentially avoid this kind of hassle.

The ZODB provides the flexibility and Generations could work well for future updates so this looks very good but how efficient is it if 15% of the attributes have data and 85% do not?

I have also been experimenting with hybrid pickle / rdb storage so that the attributes that will receive the most attention are stored as fields and the full record is stored as pickle that is unpickled for views and data entry.

In any case. Thought I'd ask since I am concerned about the efficiency of storage and speed of access both. If rdf access was fast then it would be great but this had not been the case. I just thought there may be some other ideas on this or someone could advise on the efficiency of ZODB when in some cases, uses will be selective about which attributes are important to them.

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